1. Fields of the Invention
The present invention relates to a manufacturing method of metal matrix composites (MMC), especially to a manufacturing method in which friction stir welding (FSW) is applied to produce MMC.
2. Descriptions of Related Art
In recent years, FSW technique that joins light-weight aluminum alloy and magnesium alloy has made great progress. The technique is applied to weld and join the metal plates by stirring the materials in solid state temperature range and welding them together using a stir tool driven by milling machines, multi-axis machine centers, or robots. The technique overcomes shortcomings of conventional electric arc welding such as low welding speed and reduced strength of weld metal and becomes an innovative method for welding light weight alloy. The FSW has been applied to manufacturing of components of vehicles, boats, and aircraft structures and skins.
The addition of micron-scale reinforcement particles such as Al2O3 or SiC a particulate into aluminum alloy matrix to form reinforced MMC has been developed for more than thirty years. After particulate reinforcement, the mechanical properties of metal matrix alloy are improved effectively so that MMC has been applied to components requiring higher strength and light weight. Most of MMCs have been applied in aviation or aerospace industry as structural material of flight vehicles. The most common way of adding Al2O3 or SiC reinforcement particles is smelting. The particles are added into a liquid-phase material during the smelting process and are further stirred during the solidification process. Other than this process, there is hardly further technical information regarding adding particles into aluminum alloy for producing MMC.
There are three main problems in welding the particulate reinforced MMC:
1. Arc welding of the particulate reinforced MMC: after electric arc welding, particles of the MMC exhibit a non-uniform distribution in weld metal. Also particle loss may occur. Thus the strength of the weld metal is dramatically reduced due to lacking of reinforcement particles. Thus, the welding of MMC has been always a tough issue.2. Uniform distribution of reinforcement particles in base material of MMC:during the smelting process, the addition of Al2O3 or SiC reinforcement particles into molten metal may result in a non-uniform distribution or formation of clusters. The uneven distribution of reinforcement particles in MMC causes heterogeneity and anisotropy of maternal. Thus how to improve the uniformity of particles distribution in MMC is also an important issue.3. metallurgical bonding between reinforcement particles and base material:most of the reinforcement particles are Al2O3 or SiC ceramic powder of irregular shape with particle diameter ranging from 10 μm to 50 μm. These reinforcement particles are embedded in aluminum alloy base material without metallurgical bonding with the matrix. The composite matrix is achieved merely by adhesion between base material and particles. The lack of the metallurgical bonding between particles and base material not only restricts the reinforcement effect but also reduces the fatigue strength of the MMC due to formation of microcracks caused by detachment of the particles with the base material. Thus how to enhance the bonding strength between the particles and the base material is a substantial issue. Previous studies have been conducted to evaluate the feasibility of FSW applied to MMC. It was found that the hardness and the strength of weld metal significantly decreased even the reinforcement particles are distributed uniformly in weld metal of the MMC. Thus the reinforcing effect of the particles needs to be further improved.